The present invention relates to a centrifugal fan.
A centrifugal fan disclosed in Japanese Utility Model Application Publication No. 7-14192 (JPU7-14192A) comprises an electric motor, an impeller and a volute casing. The impeller includes a plurality of blades and is fixed to a rotary shaft of the electric motor to rotate therewith. The casing includes therein a scroll air passage extending from a suction port to a discharge port, and is configured to receive the impeller in the air passage. The scroll air passage is formed such that the sectional area of the scroll air passage gradually increases toward the discharge port in the vicinity of the discharge port. A tongue portion is provided at the casing. The tongue portion projects into the air passage in the vicinity of the discharge port to form a narrowed air passage portion within the air passage. The projecting length of the tongue portion into the air passage changes stepwisely along an axial direction of the rotary shaft.